Apparatus for the direct reduction of iron ore

ABSTRACT

A method and apparatus for the direct reduction of iron ore are disclosed. A mixture of iron ore, solid carbonaceous fuel and, if sulfur is present, calcined limestone or dolomite, are used. The carbonaceous material can be cellulosic material (wood waste, paper, particularly municipal trash, garbage, etc.), charcoal, or coal (preferably sub-bituminous coal or lignite). The above feed is continuously charged into a gasification and initial reduction zone of a shaft furnace which is partitioned partially from the remainder of the furnace. Oxygen and hot steam are introduced into the upper portion of the said zone. Partial combustion or pyrolysis of the fuel and reaction with steam take place, producing reducing gas which initiates reduction of the iron ore. Ore and gas flow downwardly through conduits into the final reduction zone. Meanwhile, hydrogen-enriched reducing gas is introduced in the middle of the final reduction zone. Top gas is withdrawn from the upper open space of the final reduction zone, drawing reducing gas downwardly from the gasification and initial reduction zone and upwardly through the final reduction zone. In addition, top gas is withdrawn from the bottom of the final reduction zone. Reduction of the iron ore to sponge iron is completed in the final reduction zone. A portion of the withdrawn top gas is cooled, purified of dust, carbon dioxide and sulfur and dehumidified, then introduced near the bottom of the shaft furnace. It ascends, cooling and carburizing the sponge iron descending from the final reduction zone and becoming heated. The cooling gas is then withdrawn from the bottom of the final reduction zone. A special form of discharge grate discharges iron successively from different areas, so as to produce agitation and mixing.

This is a division, of application Ser. No. 879,250, filed Feb. 21, 1978now U.S. Pat. No. 4,160,663.

INTRODUCTION

This invention is directed to the so-called solid fuel "directreduction" of iron ore, i.e., the reduction of the ore at temperaturesbelow the melting point of iron, producing what is called "sponge iron".This product, as its name implies, has a somewhat porous structure andcontains small amounts of unreacted iron oxide, iron carbide and freecarbon. When it is melted in a steel furnace the carbon completes thereduction of the iron oxide.

BACKGROUND

Direct reduction processes have come into increasing prominence inrecent years for several reasons. They can be economically built tosmaller capacities than blast furnaces. They are better adapted tointermittent operation than blast furnaces. They are more versatile infuels than blast furnaces, not requiring the high priced coke which isnecessary for the latter. Suitable coking coals are not widelydistributed and are becoming increasingly difficult and expensive toobtain.

The direct reduction process is primarily a reduction by gases, usuallyhydrogen and carbon monoxide. In the most widely used processes,according to the literature, a mixture of these gases is produced by"reforming" natural gas by reaction with steam and oxygen. In some casesother hydrocarbon gases or light petroleum fractions have been used.

As is well known, however, natural gas is becoming increasinglyexpensive and future supplies are in doubt. There is, therefore, anincentive to use other fuels.

One commercial or developmental process utilizes coal as the source ofreducing gases. A mixture of coal and iron ore is introduced into theupper end of a huge inclined rotary kiln. Air is introduced at variouspoints along the kiln, producing partial combustion of the coal,elevating the temperature and producing carbon monoxide, which acts asthe reductant.

In another proposed process, coal is to be gasified by reaction withoxygen and steam, producing a mixture of carbon monoxide and hydrogen,which is to be used as the reducing gas. In that process thegasification of the coal takes place in either a dilute phase orfluidized bed gasifier and the gas is utilized in a separate reductionfurnace.

An extremely large number of United States patents have been granted ondirect reduction processes. Therefore, only those which appear mostpertinent to this invention will be discussed.

Cavanaugh U.S. Pat. No. 3,427,013 is directed to a "low temperatureblast furnace". A mixture of ore with coal, coke, or lignite isintroduced into the top of a shaft furnace. Heated air, in a quantitysufficient to cause only partial combustion of the fuel, is introducedat several points in the upper portion of the furnace, producing carbonmonoxide, which is considerably diluted by the nitrogen of the air. Theore is successively heated by the combustion of the fuel and reduced bythe carbon monoxide. In the lowermost section of the shaft the"metallized ore" is cooled by indirect heat exchange with the incomingair. The furnace operates under super atmospheric pressure, with wastegases being withdrawn primarily from the top of the furnace. Some,however, flows downwardly and out the bottom, serving as an air seal.

This patent does not disclose operation under conditions such as toproduce hydrogen, which is recognized as a more effective reducing agentthan carbon monoxide, and discloses no recycle of the off-gases, whichcontain a large proportion of nitrogen.

Nemeth U.S. Pat. No. 3,853,538 discloses a process in which coal orlignite (the emphasis being on the latter) is gasified in a separategasifier by partial combustion with oxygen, "little or no steam" andvery little nitrogen. The gas, said to be primarily carbon monoxide andhydrogen, is desulfurized and introduced into the lower portion, butabove the bottom, of a shaft furnace. The off gas is scrubbed and thecarbon dioxide is removed. A portion of the cooled and purified gas isintroduced into the bottom of the shaft furnace to cool the sponge iron.Another portion is mixed with the reducing gas either in the gasifier orbetween the gasifier and the desulfurizer.

The use of a separate gasifier complicates the apparatus and increasesheat loss.

Galluser U.S. Pat. No. 2,786,747 discloses a process in which ore mixedwith coal or coke is introduced into the top of a shaft furnace, whilesteam is introduced near the midpoint. THe central portion of thefurnace is heated electrically to a temperature such that the steamreacts with the coal or coke to produce carbon monoxide and hydrogen andto cause reduction of the iron ore. A temperature of 950° C. isemployed, which is below the melting point of carburized iron. A mixtureof hydrogen, water, carbon monoxide and carbon dioxide is withdrawn fromthe top of the furnace. In the upper portion of the furnace they serveto preheat the iron ore and are partially cooled. They are then furthercooled, the carbon dioxide is removed and the other gases are recycledto the bottom. Rising through the descending iron ore, they cool it andbecome heated. They then take part in the reduction of the iron ore.

This process relies on the use of electrical heating, which isexpensive.

None of these patents, and no literature of which I am aware, disclosesthe use of cellulosic material, such as wood waste, municipal trash, orgarbage as the fuel in a modern direct reduction process. Charcoal,derived from wood, was, of course, used for centuries in the reductionof iron ore before the introduction of coke.

SUMMARY OF THE INVENTION

The object of this invention is to provide a direct reduction processutilizing solid fuel which is versatile as to the fuel used andeconomical as to capital investment. The process is carried out in afixed shaft furnace in which gasification of the fuel and reduction ofthe ore take place.

Ore and solid fuel, which may be coal (preferably sub-bituminous coal orlignite), charcoal, or any cellulosic material (wood waste, paper,municipal trash, garbage, etc.), is fed into the top of a shaft furnacein which three zones exist. In the uppermost zone, the gasification andinitial reduction zone, the fuel is gasified by the controlledintroduction of oxygen (about 98% pure, i.e., substantially free ofnitrogen and other inert gases), and steam. Regenerated top gas is alsointroduced and burned. Conditions are controlled to produce gases whichare predominently CO and H₂, diluted with CO₂ and steam, whilepreheating and initiating reduction of the iron ore.

The ore, fuel and gases descend co-currently into a second zone, thefinal reduction zone. Final reduction of the ore takes place in thiszone. Hot hydrogen-enriched reducing gases are introduced in the middleof the second zone.

Off gases are removed from the upper portion as well as from the bottomof the second zone, cooled, purified of dust and carbon dioxide and, ifnecessary, sulfur. About 30% of the purified gases (essentially H₂ O, COand H₂) is mixed with air and burned to produce hot steam and providepower to extract oxygen from air. The balance of the gases is dividedinto two portions. One portion is to be enriched with H₂ by thecatalytic water shift reaction. It is this enriched H₂, CO, CO₂ togetherwith very hot steam which is introduced into the above-mentioned middleof the second zone. The other portion of the clean top gas isdehumidified and introduced near the bottom of the furnace above andbelow the grate for the cooling and carburizing to be described later.

The descending sponge iron from the second zone enters a third coolingand carburizing zone where it descends counter current to the risingrecirculated and dehumidified gases described in the previous paragraph.These gases serve to cool and carburize the sponge iron descending fromthe second zone, becoming heated in the process. They ascend to thebottom of the second zone and are sucked off to be regenerated.

Sponge iron is discharged from the third zone by a moving grid, whichdelivers material alternately from spaced portions. This producescirculation and mixing in the final reduction zone and cooling andcarburizing zone.

An amount of fuel considerably in excess of stoichiometric is employedso that carbon is discharged together with the sponge iron. The excesscarbon in the iron mixture encourages the carburization of the iron andinsures that strongly reducing conditions are present throughout thefurnace.

DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a diagrammatic illustration of the flow system of myinvention.

FIG. 2 is a top plan view of the discharge grate 12, FIG. 1.

FIG. 3 is a bottom plan view of the discharge grate.

FIG. 4 is a vertical section through the discharge grate, showingdischarge from one portion of the grate.

FIG. 5 is a section like FIG. 4, but showing discharge from anotherportion of the grate.

DETAILED DESCRIPTION

Referring to the drawings, the process is carried out in a wellinsulated fixed, vertical shaft furnace 2 provided with gas seals 1 and13 at the top and bottom, respectively. Within the furnace is apartition 6 containing discharge conduits 5. Near the bottom is adischarge grate 12, which will be described in detail later. A gasburner 3 is located above partition 6. A number of annular ribs 14 aboutthe inner periphery prevent leakage of ascending or descending gas pastthe ore.

Other inlets and outlets will be described later.

A mixture of granulated or pelleted iron ore and solid carbonaceous fuelis introduced through seal 1, which is supplied with carbon dioxideextracted from top gas.

In operation, the interior of shaft furnace 2 may be considered dividedinto three zones, the gasification and initial reduction zone A abovethe partition 6, the final reduction zone B in the portion of the spacebetween partition 6 and top gas exhaust system 10, and the cooling andcarburizing zone C in the lower portion of the furnace. The process asit proceeds in these zones will now be described.

A. The gasification and initial reduction zone

A hot mixture of steam an oxygen is introduced at 4. The oxygen shouldhave a purity of at least 98%, i.e., be substantially free of nitrogenand other inert gases. In at least the initial stages of operation,combustible gas, primarily carbon monoxide and hydrogen, are also fed toburners 3. Conditions are controlled to produce a temperature in theapproximate range 750° C. to 950° C. Burning of the gas in burners 3 andthe thermally insulated wall of the shaft help to maintain thetemperature. The temperature is controlled by regulating the ratio andtotal amount of the steam and oxygen supplied at 4 and the combustiblegas supplied to the burners 3. A water spray may be introduced intoburners 3 if the temperature should become excessive. The solid fuel issubjected to partial oxidation, pyrolysis, and reaction with steam,producing a gas containing high proportions of carbon monoxide andhydrogen. Pressure at 4 is maintained at about 21/2 atmospheres,absolute, and pressure at burners 3 is at about 2 atmospheres, absolute.The gas seal 1 and the resistance to gas flow of the material inconduits 5 make this possible.

The gas, partially reduced ore, and residual pyrolytic fuel (principallyin the form of carbon) flow downwardly into zone B, i.e., the finalreduction zone.

B. The final reduction zone

Additional hot steam and hydrogen-enriched CO-CO₂ mixture is distributedby manifold 9 to openings distributed about the periphery of furnace 2at a pressure of about 2 atmospheres, absolute. This is the approximatemidportion of zone B. Top gases are withdrawn both at 7 and 10 at apressure of about 1 atmosphere, absolute. In this zone more of the fuelis gasified and reduction of the iron ore by the gas continues towardcompletion. Temperatures of 750° C. to 950° C. are preferably maintainedin this zone. The higher the temperature, the more rapid will be thereduction, but it should not exceed a point well below the melting pointof the ash or other solids. This lower temperature will prevent theformation of slag adhering to the inside wall of the shaft.

The reactions occurring in zones A and B will now be discussed.

In zone B the fuel will be largely in the form of carbon and thereactions between the carbon, hot steam, carbon dioxide and oxygen maybe represented as follows:

    C+O.sub.2 →CO.sup.2                                 (1)

    CO.sub.2 +C→2CO                                     (2)

    C+H.sub.2 O→CO+H.sub.2                              (3)

In zone A the situation is more complicated and depends on thecomposition of the fuel. If it is coke or charcoal the reactions will bethe same as given above for zone B. As the hydrogen content of the fuelis increased, however, other and more complicated reactions come intoplay, particularly in the case of cellulose, which makes up the majorportion of wood waste, municipal trash, garbage, etc.

Pyrolysis of cellulose with the production of carbon and volatiles alsotakes place. The volatiles react promptly with oxygen and steam, whilethe carbon moves down into zone B.

As stated above, an amount of solid fuel considerably in excess ofstoichiometric is employed and a considerable amount of carbon isproduced by pyrolysis and is discharged with the reduced iron.

The gases produced may have approximately the following composition.

    ______________________________________                                        (Reducing gas)      H.sub.2 - 29%                                             (Reducing gas)      CO - 64%                                                                      N.sub.2 - <1%                                                                 N.sub.2 O - <2%                                                               CO.sub.2 - 4%                                                                 CH.sub.4 - <0.5%                                          ______________________________________                                    

It is, however, variable and can be controlled. It is a function of thekind of carbonaceous material, the reaction temperature in the furnace,etc. However, the proportion of actual reducing gas, i.e., H₂ plus CO,should be more than 90%.

The reaction between the iron ore and the reducing gas is enhanced bythe intimate contact between the iron ore and the solid materials fromwhich the reducing gas is generated, the superatmospheric pressure ofthe gas and the continually shifting mass. The shifting is enhanced bythe method of discharge, which will be described later.

The reduction of the ore involves the following reactions.

    Fe.sub.2 O.sub.3 +3H.sub.2 →2Fe+3H.sub.2 O          (4)

    Fe.sub.2 O.sub.3 +3CO→2Fe+3CO.sub.2                 (5)

    FeO+H.sub.2 →Fe+H.sub.2 O                           (6)

    FeO+CO→Fe+CO.sub.2                                  (7)

C. The cooling and carburizing zone

In the region below exhaust system 10 the reduced iron is cooled bycontact with a dehumidified reducing gas in order to prevent reoxidationupon contact with the air and H₂ O. Dried and cool reducing gas,consisting essentially of carbon monoxide and hydrogen, is introducedunder a pressure of about 3 atmosphere, absolute, above and belowdischarge grate 12 at 11 and 8, respectively, and flows upwardly throughthe descending sponge iron. Then the cooling gas is withdrawn at 10together with the top gas from zone B. In the upper, hotter portion ofzone C, a carburizing reaction takes place between the iron and thereducing gas in accordance with the following reaction.

    3Fe+CO+H.sub.2 →Fe.sub.3 C+H.sub.2 O                (8)

A certain amount of the iron carbide is desirable in the steel makingprocess. The iron carbide will serve to reduce iron oxide remaining inthe iron. One kg of carbon as iron carbide will produce about 6 kg ironfrom FeO.

In the direct reduction process, the ratio of metallic iron to totaliron content in the finished product is termed the degree ofmetallization, which is commonly expressed as a percentage. Because ofthe capacity of the iron carbide to reduce residual iron oxide in anelectric furnace, the sum of the percent metallization plus six timesthe percent carbon is termed the "equivalent metallization". Carburationduring the cooling step permits a lower degree of metallization, andtherefore a higher over-all production output, in the reducing zones,while still maintaining the desired equivalent metallization of about98%.

The discharge through the grate 12 is carried out in a particularmanner, which will now be described.

The grate 12 is formed of two elements, as shown in FIGS. 2, 3, 4 and 5.

It includes an upper fixed grate 24 and a lower rotatable grate member26. The lower grate section 26 carries a circular rack 28 which mesheswith a pinion 27 driven by shaft 29. The upper grate member 24 containsa series of openings 23 and the lower grate member 26 contains a seriesof openings 25 which are spaced differently than openings 23. Forsimplicity, I have shown only four openings 23 and three openings 25.Thus only one opening 25 will be in registry with an opening 23 at anygiven time. By rotating or oscillating plate 26 about its axis byturning pinion 27, different openings will be in registry at differenttimes. Thus, in FIG. 4, an opening 25 is in registry with one opening23, while in FIG. 5 an opening 25 is in registry with the diametricallyopposite opening 23. The successive discharge from different portionscause a shifting and mixing of material in final reducing zone B andcooling and carburizing zone C.

The discharged material consists of carburized sponge iron, ash andcarbon. The two latter components are separated from the sponge iron,e.g., by screening and/or magnetic separation.

If the ash and carbon are sufficiently different in physical properties,the carbon is desirably separated and recycled to inlet 1. If separationis not feasible but the quantity of ash is small, a portion of the ashand carbon may be discharged and the remainder recycled. It isundesirable, however, to recycle a large quantity of ash.

Limestone or dolomite may be added with the fuel if it containsappreciable quantities of sulfur. While the raw limestone or dolomitemay be used, I prefer that it be previously calcined and the resultinglime or calcined dolomite added to the charge. This avoids thegeneration of additional carbon dioxide within the furnace. The presenceof carbon dioxide tends to suppress the reduction of the iron ore bycarbon monoxide. The sulfided lime is separated from the iron ore byscreening, gravity and/or magnetic separation, and discarded.

THE GAS CIRCUIT

As has been mentioned above, hot steam and oxygen are introduced at 4into the gasification and initial reduction zone A at a pressure of 21/2atmospheres, absolute, and combustible gas is introduced in burner 3under a lower pressure of 2 atmospheres, absolute. Hot steam andhydrogen-enriched CO--CO₂ gas at 2 atmospheres, absolute, are introducedat 9. A cooling gas of dried H₂ and CO is introduced at 8 and 11 atabout 3 atmospheres, absolute. The lowest pressures in the shaft furnaceare at 7 and 10, where top gas is withdrawn at a pressure of about 1atmosphere, absolute. This pressure gradient causes a flow of thereducing gas produced in zone A together with the burning gas fromburner 3 downwardly, cocurrent with the iron ore and carbon, throughconduits 5. It also produces a flow of the hot steam andhydrogen-enriched CO and CO₂ upwardly and downwardly through zone B, andflow of the dehumidified reducing gases from 8 and 11 upwardly throughzone C. This gas has become heated and is withdrawn at 10.

The gas withdrawn at 7 and 10 is then subjected to various treatmentsand recycled. This will now be described.

First the gas is passed in indirect heat exchange with water in wasteheat boilers or heaters 16. (Because of the differences in temperatureand pressure, it is preferable to use separate units for the gas from 7and that from 10.) The hot water produced is used as feed to the hightemperature boiler to be subsequently described.

The gas from heaters 16, which is still warm, is scrubbed with water inwasher 17 to remove dust. It is then delivered to acid scrubbing tower18, where acidic gases (CO₂, COS and H₂ S) are removed. Lime solution,alkali, or alkali carbonates may be used for this purpose, but I preferto use Benfield and diethanol amine (DEA) since the solutions can beregenerated by boiling with steam and recycled, liberating CO₂. Thesteam is from the high temerature burner and boiler 19. Sulfur can berecovered from the gases as elemental sulfur by Claus technology. TheCO₂ is compressed and supplied to gas seals 1 and 13.

The cleaned gas from 18 is then divided. A portion is dehumidified at 21and returned to cooling gas inlets 8 and 11. Another portion is suppliedas feed to burners 3. Still another portion is supplied with air to hightemperature boiler 19 where it is burned at a temperature of 1500°-1700°C. to remove odor before exhaust to air through stack 23 and to generatesuperheated steam. A portion of the steam may be used to furnish powerto oxygen separation plant 22 which liquifies air and distills it toseparate oxygen from nitrogen and other inert gases. This oxygen,together with more of the steam produced at 19, is fed to inlet 4 ofshaft furnace 2. The last portion of the reducing gas, after acid washer18, is enriched with hydrogen, which is stronger in reducing power thanCO. This hydrogen enrichment is accomplished in reactor 20 by the watershift conversion:

    CO+H.sub.2 O (hot steam).sup.catalyst H.sub.2 +CO.sub.2    (9)

This mixture of hot steam and hydrogen-enriched gas, containing CO andCO₂, is introduced at the middle (9) of final reduction zone B.

ENVIRONMENTAL ASPECTS

The only gases released to the atmosphere are the combustion gasesreleased to stack 23 and the carbon dioxide from seals 1 and 13. Sincethe top gas has been purified before it is burned, these combustiongases will be almost entirely carbon dioxide, water vapor, nitrogen andexcess oxygen from the combustion air. The high combustion temperatureof 1500°-1700° C. will burn off the objectionable odor, if any. Dust isremoved from the gas by washer 17. The water can be clarified andrecycled. There should, therefore, be very little pollution of air orbodies of water.

Municipal trash and garbage create a problem for the nation from anenvironmental standpoint. By this invention, not only is the problemreduced, but this waste material is put to good use in reducing ironore, thereby conserving natural gas for other uses. The capability ofusing municipal trash and garbage as fuel, becuase of the design of thefurnace, is an important feature of my invention.

SPECIFIC EXAMPLE

A typical furnace is about 10.0 m in height and has a diameter of about7.6 m at its midpoint. It is slightly tapered to compensate for thereduction in volume as the solid fuel is gasified.

It will be noted that the gasification and initial reduction zone A is asubstantially closed chamber, separated from the remainder of shaftfurnace 2 by partition 6. It is in this chamber that the majorgasification reaction takes place. Because it is an essentially confinedzone, and isolated from the remainder of the shaft furnace, the supplyof hot steam and O₂ (98% pure) will, together with the burning of theregenerated top gas, as well as water spray, control the temperature inthe zone A and, in turn, the extent of pyrolysis of the solidcarbonaceous fuel. Owing to this feature, it can accommodate almost anytype of solid carbonaceous fuel. A particularly advantageouscharacteristic is the ability to utilize cellulosic material, such aswood waste, municipal trash and garbage, as described above.

In theory, one ton of cellulosic material should be sufficient to reducethree to four tons of iron ore. However, as noted above, it is desirableto employ an amount considerably in excess of stoichiometric. Thisinsures that strongly reducing conditions are present at all times andthat sufficient heat is produced to carry out the gasification andreduction reactions.

Accordingly, in a preferred embodiment, equal tonnages of municipaltrash or garbage or chipped wood and ore granules are continuouslycharged to furnace 2 through gas seal 1. In zone A, a large proportionof the carbonaceous fuel is gasified and the remainder is converted tocarbon. The iron ore and carbon descend through conduits 5 into finalreducing zone B. In this zone, the iron ore may constitute 70% of themass. The bulk density of the ore-carbon mixture in the final reducingzone B may be taken as about 2600 kg/m³, of which about 1800 kg will beore. Assuming a diameter of 7.6 m, a zone height of 3 m, and a residencetime of 3 hours in the zone, the throughput could be about 2800tonnes/day of ore, or about 2000 tonnes/day of metal.

I claim as my invention:
 1. Apparatus for the direct reduction of ironore comprising(a) a fixed shaft furnace, (b) a partition extendingacross the upper portion of said shaft furnace, (c) at least one conduitextending downwardly from said partition, (d) means for feeding ore intosaid shaft furnace above said partition, (e) a discharge grate extendingacross the lower portion of said shaft furnace, (f) means forintroducing heated oxygen, substantially free of inert gases, and steamabove said partition and for introducing reducing gas between saidpartition and said discharge grate. (g) means for withdrawing top gasimmediately below said partition, (h) means for cooling, purifying anddehumidifying said top gas and introducing a portion of said cooled andpurified top gas into said shaft furnace adjacent said discharge grate,2. Apparatus as defined in claim 1 wherein said discharge gratecomprises(a) an upper grate section and a lower grate section, each ofsaid grate sections having openings therethrough, the openings in theupper grate section being spaced differently than those in the lowergrate section, and (b) means for moving one of said grate sectionsrelative to the other about a central axis, whereby material in saidshaft furnace may be discharged successively from different portions ofsaid discharge grate.